NASA - 2004 and 2012 Transits of Venus

Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Indeed, only six such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874 and 1882). The next two transits of Venus will occur on 2004 June 08 and 2012 June 06.

The principal events occurring during a transit are characterized by contacts. The event begins with contact I which is the instant when the planet's disk is externally tangent with the Sun. The entire disk of the Venus is first seen at contact II when the planet is internally tangent with the Sun.

During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arc-min/hr. At contact III, the planet reaches the opposite limb and is once again internally tangent with the Sun.

The transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Greatest transit is the instant of minimum angular separation between Venus and the Sun as seen from Earth's geocenter.

Figure 1 (Low Res or High Res) illustrates the geocentric observing geometry of each transit across the Sun (celestial north is up). The 2004 transit crosses the Sun's southern hemisphere while the 2012 event crosses the northern hemisphere.

The position of Venus at each contact is shown along with its path as a function of Universal Time. Each transit lasts over six hours. The apparent semi-diameters of Venus and the Sun are 29 arc-seconds and 945 arc-seconds respectively.

This 1:32.6 diameter ratio results in an effective 0.001 magnitude drop in the Sun's integrated magnitude due to the transit. Geocentric contact times and instant of greatest transit appear to the left corners of figure 1 (Low Res or High Res).

NASA - 2004 and 2012 Transits of Venus